Nanoparticles (NPs) of various metals, in particular, copper, silver, gold, zinc, and titanium, are now considered promising and multifunctional components for biomedical applications. For the formation of nanoparticles, a “green” synthesis method has been developed based on the use of plant extracts to initiate nanoparticles. This method is considered simple and safe as it involves the use of extracts from well-studied plants. The main condition for the possibility of “green“ synthesis of metal nanoparticles is the presence of the similar activity in the extracts. The possibilities of using Calendula officinalis L. hairy root extracts for the synthesis of silver nanoparticles (AgNPs) and its dependence on flavonoid concentration in the extracts were studied in this work. The extracts obtained with the use of 70% ethanol had the highest reducing activity in comparison with the aqueous extracts. Reducing activity of the extracts correlated with the concentration of flavonoids. The presence of nanoparticles of different sizes was confirmed by using transmission electron microscopy. Colloid solutions of AgNPs obtained using the extract with a higher content of flavonoids had significantly higher absorption values in the range of 420–440 nm, which is characteristic of AgNPs. An increase in absorption over time (up to two weeks) indicates long-term preservation of reducing activity in the mixture with AgNO3. The smallest AgNPs (0.33… 7 nm) were formed when an aqueous extract was used, and the largest ones (up to 41.83 nm) with an extract obtained with 96% ethanol. Thus, it is the aqueous C. officinalis extract that should be chosen if it is necessary to obtain silver nanoparticles of small size.
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